1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a spread illuminating apparatus with a sidelight system that includes a light guiding plate in which a light source is disposed on a light incident face of the light guiding plate, so that spread illuminated light is emitted from an emission portion of the light guiding plate.
2. Description of the Related Art
As a lighting unit of a liquid crystal display panel, a spread illuminating apparatus with a sidelight system (backlight) where small and environmentally high-suitable LEDs are arranged along a light incident face of a light guiding plate has been widely adopted mainly in a field of small portable information apparatuses such as a mobile phone. In such a spread illuminating apparatus, luminance irregularities, that is, the generation of bright areas in front of each of the LEDs as well as the generation of dark areas between the LEDs, have been noticed that these luminance irregularities tend to occur near the light incident face of the light guiding plate. Conventionally, in order to fix these luminance irregularities, a spread illuminating apparatus with a light incident prism has been proposed, the light incident prism being arranged at the light incident face of the light guiding plate for light diffusion. See, for example, Japanese Patent Application Laid-Open No. 2002-42534.
Along with recent advanced output performances of LEDs, the number of LEDs arranged at light incident faces of a light guiding plate has been reducing in the spread illuminating apparatus. In this case, since an interval between the LEDs adjacent to each other will become longer, dark areas tend to be generated near the light incident face of the light guiding plate. In order to reduce such dark areas, the contour of a light incident prism may be adjusted thereby allowing further light to direct toward the dark areas (that is, diagonally to the front of the LEDs). However, problems to be described hereinbelow may arise anew.
FIG. 4 is a top view illustrating a spread illuminating apparatus 100 that includes a light guiding plate 110 and two LEDs 101a and 101b arranged at a light incident face 102 of the light guiding plate 110. In the light incident face 102, a light incident prism (not shown) is formed. In the spread illuminating apparatus 100, if an interval p between the adjacent LEDs 101a and 101b becomes relatively longer, the contour of the light incident prism needs to be adjusted to diffuse light at further wider angle. With the adjusted contour of the light incident prism, a sufficient amount of light will advance from the LEDs 101a and 101b toward a region (portion A between the adjacent LEDs) that tends to become the dark areas. As a result, for example, light emitted from the LED 101a and diffused by the light incident prism so as to travel into the light guiding plate 110 includes: 1) light (M) that arrives at the portion A and then emitted therefrom; and 2) light (M′) that passes through the portion A, arrives at a front region (portion B) of the adjacent LED 101b, and then emitted therefrom. Since the portion B is an area that tends to be originally bright by light emitted from the LED 101b, the brightness of the portion B should be further advanced by light emitted from the LED 101a consequently creating a so-called hot spot. Accordingly, based on arts provided by the spread illuminating apparatus 100, the luminance irregularities should be further increased.